Angle-modulation wave receiver



Jan. 7, 1947.

W. R. KOCH ANGLE-MODULATION WAVE RECEIVER F1198 Nov, 18, 1944 2Sheets-Sheet 1 INVENTOR ATTORNEY Jan. 7, 1947. w H- 2,413,977

ANGLE-MODULATION WAVE RECEIVER Filed Nov. 18, 1944 2 Sheets-Sheet 2Tlclju.

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4r y a 2'2 ATTO'RN EY Patented Jan. 7, 1947 attain ANGLE-MODULATION WAVERECEIVER l/Vinfield H. Koch, Haddonfield, N. J., assignor to RadioCorporation of America, acorporation of Delaware Application November18, 1944, Serial No. 564,047

12 Claims.

My present invention relates to receivers of frequency modulated (FM) oramplitude modulated (AM) carrier waves, and more particularly to radioreceivers utilizing novel FM detector circuits.

An important object of my present invention is to provide a noveldetector of angle modulated carrier waves wherein each of a pair ofrectifier tubes concurrently functions as a load impedance for itscompanion tube.

Another important object of my invention is to provide an FM-AMdetection network wherein no actual detector output load resistors areemployed, but instead the detector tubes per se are employed for thatpurpose.

Another object of my invention is to provide a switchless FM-AM detectornetwork wherein triode detection is utilized, and each triode serves asthe output load element for another triode.

Still other objects of my present invention are to improve generally theefficiency of FM-AM receivers, and more especially to provide economicaldetector circuits for FM receivers.

Still other objects of my invention will best be understood by referenceto the following description, taken in connection with the drawings, inwhich I have indicated diagrammatically several circuit organizationswhereby my invention may be carried into effect.

In the drawings:

Fig. 1 shows, in partial schematic form, an FM- AM receiver employingone embodiment of my invention;

Fig. 2 shows a modification of the detector network of Fig. 1;

Fig. 3 illustrates the FM detection characteristic of the detectorcircuit of Fig. 2; and

Fig. 4 shows a further modification of the FM detector circuit.

Referring now to the accompanying drawings, wherein, like referencecharacters in the different figures designate similar circuit elements,Fig. 1 shows an illustrative receiving system embodying a demodulatornetwork adapted to provide audio frequency voltage in response to FM orAM signal reception. The receiver circuits prior to the demodulator areschematically represented. Those skilled in the art of radio receptionare well acquainted with the nature of the circuits customarily employedin multi-band receivers. While my invention is readily adapted for FMand AM reception on respective bands of 40 to 50 megacycles (me) and 550to 1700 kilocycles (kc), it is to be clearly understood that theinvention is not limited to such frequency bands.v The 40 to FM Waves.

50 me. band is presented by way of illustration, since it is the FMbroadcast band presently assigned to such transmission. The 550 to 1700kc. band is the present AM broadcast band assigned to transmission of AMsignals.

It will further be understood that in the present description andfollowing claims the generic expression angle modulated is intended toinclude frequency modulation or phase modulation. Froma very generalviewpoint my invention relates to a demodulator network having separateinput circuits for carrier waves of different frequencies and ofdifferent modulation characteristics.

The numerals I and 2 in Fig. 1 denote respectively different sources ofmodulated carrier waves. Source I may be the usual signal coll'ector,such as a dipole, employedfor collecting The FM carrier waves aretransmitted from FM transmitters at a center, or carrier, frequencyassigned to the particular transmitter. In the assumed FM band of 40*to'mc. the radiated. carrier wave frequency would be in that range, andwould be a wave of variable frequency and substantially uniformamplitude. As is Well known, the frequency modulation of the carrierwave would be in accordance with the modulation signals at thetransmitter. The extent of frequencydeviation of the carrier frequencyis a function of the modulation signal amplitude, while the rate offrequency deviation is dependent upon the modulation signal frequenciesper se. The permissible extreme frequency deviation in the FM band of 40to 50 Inc. is '75 kc. to either side of the carrier frequency. Thesefrequency values are purely illustrative.

Source 2 may be a suitable grounded antenna circuit employed in AMbroadcast reception. The allotted channels are 10 kc. wide in this band.In AM transmission the carrier wave is modulated in amplitude inaccordance with the modulation signals. The carrier frequency ismaintained constant in value at the transmitter. The numeral 3designates a tunable radio frequency amplifier having suitable signalselector circuits for FM and AM reception. Switching devices 4 and itare provided for separate connection of the sources I and 2 torespective selector circuits of amplifier 3. It will be understood thatwhen switch 4 is in closed position, collected FM signal energy will beapplied to selector circuits of amplifier 3 capable of selectivelyamplifying the FM signals over a band at least k'c.'wide. Upon closingof switch '5, and opening switch 4, the

same amplifier 3 will have the FM selector circuits thereof operativelyreplaced by AM selector circuits. These latter circuits will select thecollected AM signals, and permit amplifier 3 to am plify the same over a10 kc. band. Multi-band selector circuits and switching devices forsuitable change-over are well known to those skilled in the art of radiocommunication.

Assuming the system is of the superheterodyne type, as is the usualpractice at present, the converter 6 and intermediate frequency (I. F.)amplifier I will, also, be provided with suitable FM and AM signalselector circuits. At the converter 6 the FM signals will have thecenter frequency thereof reduced to a value which may be chosen from arange of 1 to 20 mc., as for example 4.3 mc. The AM signals are reducedto an I. F. of 455 kc., as an illustrative frequency value, the latterbeing a commonly employed frequency in AM broadcast receivers of thesuperheterodyne type. The I. F. amplifier I, which may consist of one ormore separate stages of amplification, will have an ultimate outputcircuit from which may be derived, at separate points thereof, theamplified FM signals or AM signals.

The selective circuits 8 and 9 are to be understood as being arranged inseries in the plate circuit of the last I. F. amplifier tube. Each ofcircuits 8 and 9 is resonated to its respective operating I. F. valuefor PM or AM reception. Thus, circuit 8 is tuned to 4.3 mc., whilecircuit 9 is tuned to 455 kc. There will be developed across tunedcircuit 8 the FM signals at the 4.3 mc. mean frequency when switch 4 isclosed. and all FM elector circuits of amplifier 3, converter 6 and I,F. amplifier I are in operative electrical connection. Conversely, whenswitch 5 is closed, and switch 4 is open, and all AM selector circuitsare in operative electrical connection, there will be developed acrosscircuit 9 the AM signals at the I. F. value of 455 kc. The impedance ofcircuit 9 is negligible at 4.3 mc. Hence, the insertion of circuit 9 inseries with circuit 8 will not affect the development of FM signalvoltage across circuit 8. Similarly, the impedance of circuit 8 isnegligible at 455 kc., and circuit 8 will not affect development of AMsignal voltage across circuit 9.

The present demodulator comprises two electron discharge devices of thetriode type. The triode I is provided with a resonant input circuit I2which is inductively coupled to the circuit 8. Triode II has its cathodeI9 established at ground potential, while its grid I 3 is connectedthrough condenser I4 to the high alternating potential side of itsresonant input circuit I5. Circuit I5 is, also, inductively coupled tothe circuit 8. The low potential side of circuit I5 is connected to thegrounded cathode I9 through the coil I6. Coil I6 is magnetically coupledto circuit 9, and condenser I8 shunts coil I6 to provide a resonantcircuit IB-I8 tuned to 455 kc.

The input circuits I2 and I5 of respective triodes I0 and I I areoppositely and equally mistuned with respect to the operating I. F.value for FM reception. In other words, if the circuit 8 has anoperating frequency FL: of 4.3 mc., then 'circuits I2 and I5 will bedetuned in opposite senses by equal predetermined frequency valuesrelative to Fe. Thus, circuit I2 is indicated as tuned to a frequency inexcess of Fe, and circuit I5 is represented as being tuned to afrequency less than Fe. For example, circuit I2 could be tuned to 100kc., more than 4.3 mc. (4.4 mc.) whilecircuit I5 could be tuned to 4.2mc. These are only illustrative values. It will be recogranged in seriesrelation so far as the space current paths thereof are concerned. Theplate 20 of triode I0 is connected to the +13 terminal (say, forexample, +200 volts) of a suitable direct current supply source. Thecathode 2I is connected directly to the plate 22' of triode I I. The lowpotential side of input circuit I2 is connected to cathode 2L Thecontrol grid 22 of trio-dc I0 is connected by direct current blockingcondenser 23 to the high alternating potential side of input circuit I2.Resistor 24 provides a direct current return path for grid 22. Similarlyin the case of detector triode II, the resistor 25 returns grid I3 tothe grounded cathode I9.

It will be seen that a unique feature of my present circuit is thecomplete absence of output or load resistors external of tubes I0 and II. The internal cathode to plate impedance of each of triodes I0 and IIis utilized as the load mpedance for the companion triode. The internalresistive impedance of triodes I0 and II are indicated by numerals 26and 21 respectively. The output resistive impedances 26 and 21 areindicated by dotted lines to show that they are the internal resistiveimpedances of the triodes I0 and I I. The junction of resistiveimpedances 26 and 21 is connected to the audio frequency couplingcondenser 28, the carrier bypass condenser 29 connecting the junctionpoint to ground. Leads 30 function to feed the modulation signals to asubsequent suitable audio output circuit.

The detectors may function as grid leak detectors, or gridcurrent-biased plate detectors. Thus, if the condensers 23 and I I wereeach assigned a value of 50 micromicrofarads (mmf.) and each ofresistors 24 and 25 a value of 200,000 ohms, the detectors would performas grid rectification devices. By assigning a value of 0.005 mmf. toeach of condensers 23 and I4 and a value of 10 megohms to each ofresistors 24 and 25, the detectors perform as grid current-biased, platerectification. devices.

During AM signal reception the I. F, signal energy produced in thecircuit 9 will be transferred to input circuit I6, I8. Each of circuits9 and I6, I8 is tuned to the operating I. F. value of 455 kc. Thecircuit I5 and the grid to cathode path of triode I I are in a seriescircuit with tuned circuit I6, I8. The circuit I5, resonant close to 4.3mc., has no appreciable effect on the series circuit, since it acts asan extremely low impedance connection at the 455 kc. frequency value.The modulation (AM) signal voltage component of the rectified I. F.energy developed across load resistive impedance 26 during grid leakdetection at tube II is applied through condenser 28 and leads 30 to thecommon modulation signal output circuit. Hence, it will be seen thatduring AM signal reception the tube II functions as the detector tubefor the AM signals at input circuit I6, I8. The output resistor in thatcase is the internal cathode to plate resistance of tube I0. Inother-words, the audio frequency voltage developed acrossresistiveimpedancezfi is utilized through'the output leads 30.

For FMsignal reception, on the other hand,'internal resistances 26' and21' are alternately used as the frequencyo f the I. F; energy deviatesrelative to its mean or center frequency value. Assume, first, thattheFM signalis at4.3 me. In such case both circuits l2 and i5 apply equalsignal energy to the respective detector tubes. Assuming that tubes Illand H are of like construction, the'resistance values of'impedances 26and 2'! will be equal. Therefore, there willbe equal voltage dropsacross impedances '26 and 21, and the potentialat junction point a willhave a predetermined normal or mean value.

Suppose; now, that the applied FM'signal energy at circuit ii deviatesin frequency towards the frequency of" circuit l2 (4.4 incl).

The current flow through the tube It} would decrease, assuming thecondenser 23 and resist tor Hare chosen to provide grid leakdetection.There would occur a relatively larger voltage drop acrossresistiveimpedance 26. This fol lows from these considerations. When asignal is applied to the grid 22, rectification makes the average gridpotential more negative relative to cathode 2|. This tends to stopelectrons from flowing to plate 28; Theelectrons start piling up on thecathode 2! thereby making it more negative relative to the plate.Therefore, the'voltage across the internal impedance 2B of the tube isgreater. This would mean thata smaller voltage drop existed acrossresistive impedance 21. Hence, the point a would be relatively.lesspositive relative to ground. If; now, the FM signal frequencydeviates towards the resonance frequency of circuit Hi, the reversetakes place. That is, the point a becomes more positive .relative toground. The sense anddegree of potential change of point a will bedependent upon the direction. and amount respectively of' frequencydeviation of the FM. signaLenergy. Itwill be noted that the audio outputcircuit is. singleended. Condenser. 29 will be chosen to bypass all highfrequency components, whether at .455; kc. orat 4.3 me.

The detection networlsof Fig.1 mayv assume. difierent forms. Figs. 2 and4, show. different modifications of the invention. InFig. 2 I have showna modification of the invention wherein for FM signal reception thereisused a. discrimi nator which is of the type. disclosed and-claimed byJohn D. Reidin U. S. Patent No. 2,341,240 granted February 8,. 1944. Inthis embodiment of the invention the AM- circuit 9- provides the inputfor. triode-detector H; The condenser. I l and resistor function toprovide grid; leak detection for AM reception. Here, again, theresistive impedance 26 acts as the: plate load for the grid leakdetector tube l I. It will be noted that the high alternating potentialside of input circuit 9, is' coupled to grid; l3'bya; series pathconsisting of .tunedf circuit'i8', condenser l4 and resonant circuit12'. Since circuits 8! and i2 are tuned: in the:vicinity' 0f4;3. mo.theyahave negligible impedance at 455-kc; Hence;v during AMjreceptionthe grid l3 is-ineflfectdirectlyv connected to the high potential sideof circuit 9. An automatic volume control '(AVC) connection 40 isr'nadeto the gridend of'resistor' 25 to provide AVC' bias for prior-gaincontrolled 6 reception, andv upon grid current flow through resistor 25,AVG biasis produced.

To detect FM signals the tuned circuits 8 and [2' are employed. Resonantcircuit 8' is connected in series with circuit 9 in the I. F. amplifieroutput circuit, and is tuned approximately to Fe (4.3 mc.). The resonantcircuit I2 is tuned somewhat above Fe. The control grid 22 of triode I9is coupled by condenser 23 to the upper end of coiLSil. The latter ismagnetically coupled to circuit 8, and has its lower end connected tothe cathode 2| of triode Ill. The

resistor '24 connects grid 22 to cathode 2 l Hence,

during FM reception triode II] has applied thereto the signal energyacross circuit 8, while triode H has applied to it the signal energyexisting in the circuit including both circuits 8 and [2. Of course,circuit 9 has no effect at FM reception, since its impedance isnegligible at 4.3 mc.

The aforesaid Reid patent fully discusses the operation of an FMdiscriminator of the type shown in Fig. 2. By virtue of thediscriminating action of circuits 8' and [2' the triodes l0 and l l willhave amplitude modulated waves applied thereto which were derived fromthe received FM signal waves. Resistive impedance 2S acts as the loadfor triode H, and develops rectified voltage thereacross. Resistiveimpedance 2i acts as the load for triode It and de- 1 velops rectifiedvoltage thereacross.

The FM signal voltage across circuit 8 is applied to grid 22 throughcoil 38. Triode grid l3 has applied to it the signal voltage across itsown input circuit. At the frequency where circuit l 2 is anti-resonantthe voltage across circuit l2 will be high, while that at grid l3 willbe very low. At a somewhat lower frequency, circuit IE will have aninductive reactance which will series resonate with the capacity betweengrid l3 and cathode l9 thereby making the voltage applied to the triodel i very high. It is thus apparent that the radio frequency voltageapplied to triod l I changes very rapidly with frequencies betweenth'ese points. At the first frequency the voltage across circuit 8 willbe high thereby causing the voltage applied to triode It) to be high. Atthe second frequency the series resonant circuit formed by circuit l2and the input capacity of triode ll will load down circuit 8 therebycausing the voltage applied to triode I!) to be low. Hence, therectified voltage output of triode. H changesin a direction opposite tothat of triode l0 as the instantaneous frequency of the applied FMsignals varies. Correct circuit design produces the desiredcharacteristic. In Fig. 3 there is illustrated the frequency re sponsecharacteristics at grid l3 (solid line curve) and grid 22 (dotted. linecurve) respectively. The cross-over frequency is at 4.3 me.

In Fig. 4 there is shown a further modification of my invention, whereinthe AM input circuit 9 is omitted and the I. F. amplifier includes theF. i. circuit 8 tuned to 4.3 Inc. The latter is coupied magnetically toresonant circuit 3i tuned to 4.3 me. The plate side of circuit 3 isconnected by lead 32 tothe midpoint ofcoil 33 of secondary circuit 3?.Control grid22 is coupled by direct current blocking condenser to oneside of circuitiii, and control gridlli is coupled by direct Fig. 4 i ofthe typedisclosed and claimed by S.

tubes. Filter; network ll isemploycd'to suppress any. pulsating:components: of thenAVC' voltage; Asthe. carrier amplitude:- increasesduring AM W. Seeley in-U. S. Patent No. 2,121,103 granted June 21, 1938.

The discriminator: circuit of Fig. 4 functions to provide at each ofgrids 22 and l 3 a signal voltage whose magnitude is the resultant oftwo signal voltages. The magnetic coupling between primary circuit iiand secondary circuit ill results in the application to grids 22 and I3ofsignal voltages in polarity opposition and in phase quadratur relativeto the primary circuit signal voltage. The direct connection of circuit8 to the midpoint of coil 33 causes signal voltages to be applied togrids 22 and i3 in like polarity and in parallel. Hence, at signalfrequency Fc each grid has applied thereto the resultant vector voltageof a phase-shifted signal voltage and a nonshifted voltage in normalphase quadrature. The resultant vector voltages at grids 22 and i3 areequal at Fe. However, for signal frequencies different from F0 (theoperating I. F. value) the resultant vectors become unequal, because theshift due to the magnetic coupling between circuits 8 and 3| changes.The respective resultant vector voltages at grids 22 and f3 wili vary insense and magnitude in response respectively to the direction and extentof signal frequency deviation.

These resultant signal voltages will cause corresponding voltage dropsacros the internal resistive impedances 26 and 21. The operation isquite similar to that described in connection with Fig. i. If desired,an AM input circuit could be added to cooperate with one of the detectortubes it or H. Of course, the triodes could be located in a single tubeenvelope, in the manner of a twin triode, if compact construction weredesired. Grid leak detection can be made to give only a small variationin audio frequency output voltage for a considerable range of inputsignal amplitudes. A reduction in noise and distortion is accomplishedin the circuits described through this type of detection. Low mu tubesare recommended where there is available sufficient signal amplitude toprovide AVC, AVC action could be secured in either of the systems ofFig, l or Fig. 4, in which case low mu tubes are preferred. For maximumsensitivity it i preferred to use high mu tubes, and will be suitablefor signals too weal: to furnish AVC bias.

While I have indicated and described several systems for carrying myinvention into efiect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationshown and described. but that many modifications may be made withoutdeparting from the scope of my invention.

What- I claim is:

1. In a detection system for angle modulated carrier waves, a pair ofelectron discharge devices having the space current paths thereofconnected in series relation, means for deriving from said waves a pairof carrier voltages each variable in amplitude, means separatelycontrolling the space current flow of each device in response to therelative amplitudes of said pair of voltages, and modulation signaloutput connections connected across the space current path of one ofsaid devices.

2. In combination with a pair of triodes, means connecting the internalplate to cathode impedances thereof in series relation, a frequencydiscriminator input circuit, separate connections from said frequencydiscriminator circuit to a respective control grid of each. of saidtriodes, and a modulation signal output circuit connected across onecfsaid internal plate to cathode im- Dcdances.

3. In combination with a pair oftriodes, means connecting the plate tocathode impedances thereof in series relation, a frequency discriminatorinput circuit, separate connections from said frequency discriminatorcircuit to a respective control grid of each of said triodes, amodulation signal output circuit connected across one of said plate tocathode impedances, a Signal input circuit tuned. to a frequencysufficiently different from the resonant frequency of said discriminatorcircuit to have negligible impedance effect on the latter, andconnections between the input electrodes of one of said triodes and saidsignal input circuit.

4. In a frequency modulation receiver, a pair of detector tubes eachincluding at least a cathode, control grid and anode, means forestablishing the cathode of one of the tubes at ground potential, meansconnecting the anode of said one tube directly to the cathode of thesecond tube, means for applying a positive potential to the anode of thesecond tube whereby the space current paths of the two tubes are inseries relation,

modulation signal output connections connected to the anode and cathodeof one of said tubes, and a frequency discriminator network havingseparate signal input connections to the control grids of said pair oftubes.

5. In combination with a source of amplitude modulated carrier waves, adetector tube having input, cathode and plate electrodes, an inputcircuit tuned to the carrier frequency coupling said source to saiddetector tube input and cathode electrodes, a second tube provided withan input, cathode and plate electrodes and having its internal plate tocathode impedance in series relation with the plate to cathode impedanceof said detector tube, means connecting the plate of the second tube toa source of positive voltage, and modulation. signal output connectionsconnected across the plate to cathode impedance of said detector tube.

6. In a demodulator circuit for modulated carrier waves, a pair oftriodes Whose respective internal plate to cathode impedances areconnected in series between a point of relatively high positive voltageand ground, means for varying the voltages of the control grids of saidtriodes in accordance with frequency deviation of a frequency-variablewave to be demodulated, and means responsive to the voltage variationsof the junction of said plate to cathode impedances for producing amodulation signal representative of said frequency deviations.

7. In a detection system for frequency modulated carrier waves, a pairof electron discharge tubes having the space current paths thereofconnected in series relation, discriminator means for deriving from saidWaves a pair of carrier voltages each variable in amplitude, controlgrid means separately controlling the'space current flow of each tube inresponse to the relative amplitudes of said pair of voltages, andmodulation signal output connections connected across the internalimpedance of one of said tubes.

8. In combination with a pair of grid leak detector triodes, meansconnecting the internal impedances thereof in series relation, afrequency discriminator input circuit, separate connections from saidfrequency discriminator circuit to a respective control grid of each ofsaid triodes, and a modulation signal output circuit connected acrossone of said internal impedances.

9. In combination with a pair of grid detection triodes, meansconnecting the internal plate to cathode impedances thereof in seriesrelation, a

frequency discriminator input circuit, separate connections from saidfrequency discriminator circuit to a respective control grid of each ofsaid triodes, an audio signal output circuit connected across one ofsaid plate to cathode impedances, an amplitude modulation signal inputcircuit tuned to a frequency sufiicien-tly different from the resonantfrequency of said discriminator circuit to have negligible impedanceefiect on the latter, and

connections between the input electrodes of solely i one of said griddetection triodes and said signal input circuit.

10. In a frequency modulation receiver, a pair of grid leak detectortubes each including at least a cathode, control grid and anode, meansfor establishing the cathode of one of the tubes at ground potential,means connecting the anode of said one tube directly to the cathode ofthe second tube, means for applying a positive potential to the anode ofthe second tube whereby the space current paths of the two tubes are inseries relation, audio frequency signal output connections connected tothe anode and cathode of one of said tubes, and a frequencydiscriminator network having respective signal connections to thecontrol grids of said pair of tubes.

11. In a receiver circuit for frequency modulated carrier waves, a pairof grid leak detector triodes whose respective internal plate to cathodeimpedances are connected in series between a point of relatively highpositive direct current voltage and ground, means for varying thevoltages of the respective control grids of said triodes in accordancewith frequency deviation of a frequency modulated wave to bedemodulated, and means responsive to the voltage variations of thejunction of said plate to cathode impedances for producing a modulationsignal representative of said frequency deviations.

12. In combination, a pair of grid leak detector tube circuits, meansconnecting the internal impedances of said detector tubes in seriesrelation, means for varying the voltages of the control grids of thetubes in accordance with received frequency modulated waves, and amodulation signal output circuit connected across the internal im'pedance of one of said detector tubes.

WINFIELD R. KOCH.

